Effect of pesticides on microorganisms involved in litter decomposition in cacao plantation in Ile-Ife, Nigeria

Osewole, Afolabi ; Muoghalu, I.Joseph (2016)

Article in Agroforestry Systems,Vol. 92,No.3

Journal

Wide spread use of pesticides by farmers in agro-ecosystems to control pests may cause the disruption of nutrient cycling by adversely affecting the organisms especially microorganisms, namely bacteria and fungi, involved in decomposition processes thus altering normal nutrient cycling pattern in these ecosystems. This study, therefore, identified and determined the dynamics of bacterial and fungal species involved in cacao leaf litter decomposition and the effect of the pesticide, Ridomil Gold 66WP on the species. This was with a view to understanding the effects of pesticides used to control pests and diseases on litter decomposition rate in cacao plantations. Cacao leaf litter decomposition was studied using litterbag method. Freshly fallen leaf litter was thoroughly mixed and divided into two groups, treated and untreated. The treated was sprayed with Ridomil Gold 66WP at the rate of the recommended field rate of 3.3 g/L and air dried. 20 g of treated and untreated litter were weighed, put in separate 15 cm × 15 cm of 1.0 mm mesh size litter bags and randomly placed on the floor of cacao plantations in Ile-Ife, Nigeria. Three litter bags of each group were randomly retrieved from the plantation on monthly intervals for each month and the content of each litterbag was weighed to determine the rate of decomposition of the litter. The litter contents of each group litter were bulked, ground and microbiological analyses carried out to isolate, characterise and identify the species of bacteria and fungi involved in cacao leaf litter decomposition. The rate of decomposition of pesticide treated cacao leaf litter (0.473) was lower than that of untreated litter (0.582). A total of 31 microbial species consisting of 13 bacterial species and 18 fungal species were isolated from decomposing cacao leaf litter during the sampling period. More fungal species (16) than bacterial species (9) were involved in untreated cacao leaf litter decomposition. Species of the genera Klebsiella, Pseudomonas, Aspergillus, Penicillium and Rhizopus were the most common microorganisms involved in decomposition of cacao leaf litter. Ridomil Gold 66WP did not have significant effect on the total heterotrophic bacterial and fungal counts throughout the sampling period though their counts in treated litter were lower than in untreated litter. There were also no significant monthly variations in both total bacterial and fungal counts. It was concluded that the pesticide, Ridomil Gold 66WP, though had adverse effect on the rate of decomposition of cacao leaf litter, it did not have any adverse effect on the species of bacteria and fungi involved in decomposition of cacao leaf litter.